A signal re-driving device, a data storage system and a mode control method are provided. The method includes the following steps. A first signal is received via a receiving circuit of the signal re-driving device. An analog signal feature is detected the receiving circuit. A first mode is entered according to the analog signal feature. The first signal is modulated and a second signal is outputted in the first mode. The second signal is sent via a sending circuit of the signal re-driving device. A digital signal feature is detected via the receiving circuit. And, the first mode is switched to a second mode according to the digital signal feature.